When You first fill your res to the nute specs on the bottles and say im in week three or so and my ppms are supposed to be reading say 500 ppm's and then I check the res say a day or two later and my ppm's drop down to say 450 or 475 or just go lower than what there supposed to be reding am I supposed to add more nutes until I get the water back up to 500 ppm's and check water every day to see what plant are eating and balance out res with more food or just let it run low until next water change in one to two weeks . thanks for any advise